1. Field of the Invention
The present invention relates to a curl correction apparatus that corrects a curl of a sheet, a sheet conveying apparatus including the same, and an image forming apparatus including the same.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2005-255288 discloses a curl correction apparatus that is disposed in the immediate downstream in a sheet conveying direction with respect to a fixing apparatus, includes a hard roller, and a soft roller pressed against the hard roller, and corrects the curl of the sheet at a nip between the rollers. At the nip between the hard roller and the soft roller, an image fixing surface of the sheet on which a toner image is fixed hits on a peripheral surface of the hard roller, and a back surface thereof hits on a peripheral surface of the soft roller. Although the sheet is deformed to an upper curl (a curl in which the sheet has a convex shape toward the lower side) toward the image fixing surface by the fixing apparatus, the upper curl of the sheet is corrected in the process of passing through the nip between the hard roller and the soft roller.
Furthermore, in the invention described in Japanese Patent Laid-Open No. 2005-255288, the nip is formed in such a manner that the hard roller bites into the soft roller by bringing the bearings of both end portions of the soft roller into contact with the bearings of both end portions of the hard roller. In this case, the hard roller and the soft roller are bent by the reaction force at the nip portion, and the axial distance between the central portions in an axial direction (a longitudinal direction) thereof increases compared with both end portions. For this reason, in the hard roller and the soft roller, a biting force between both end portions in the axial direction is large, but the biting force in central portion in the axial direction becomes smaller. In order to back up the biting force in the central portion in the axial direction by reducing a difference in the balance of the biting force, a backup roller is disposed toward the hard roller, and the backup roller applies a force to the soft roller by pressing the central portion in the axial direction of the hard roller.
However, in the configuration using the backup roller, unfixed toner attached to the hard roller may move to the backup roller and may be gradually accumulated. Moreover, the unfixed toner accumulated on the backup roller may move to the hard roller at a time to cause image failure on the sheet. Furthermore, in the hard roller, a portion hitting on the backup roller, and a portion that does not hit thereon are generated to make a temperature difference between the hitting portion and the non-hitting portion, thereby causing unevenness in the image fixed to the sheet.